1. Technical Field
The present disclosure relates to low power states for computer devices, and, more particularly, to improved power-savings for sleep mode.
2. Discussion of Related Art
In a server cluster, there may be servers that are not currently in use and thus put into sleep mode. While in sleep mode, the servers retain power to all the dual in-line memory modules (DIMMs) that are keeping a stored state active in order to turn back on faster when needed.
In order to save energy resources, many laptops and other computing systems provide the user with a capability that can place the computing system in a low-power sleep mode from which it can be awakened, or ‘resumed’, with low latency. The latency is especially short when the suspend-to-random access memory (RAM) sleep mode is selected. This mode can be designated as the ‘S3 sleep mode’, or ‘S3’, while the term ‘S4 sleep mode’, or ‘S4’ can be designated for a suspend-to-disk sleep mode.
During S3 sleep, the system state is preserved in memory, which is kept alive, in a lower-power, self-refresh mode. The wake up from S3 sleep is quite fast, since the compute node needs only to restore architectural state, and restore devices from their respective sleep modes and resume execution from the last point before suspension. The S4 sleep mode persists a system state in a disk in a special restore file, after which the entire system can be powered down. Therefore, S4 is a completely powered-off sleep state. However, since the system needs to restore its entire state from the disk on resume, it has a substantially longer resume latency as compared to S3, and, therefore, S3 is a desirable option for applications where system response latency is critical.
The availability of sleep modes is especially desirable in large-scale computing environments, such as cloud computing environments. Clouds provide tremendous compute power that, according to ample monitoring data that has been collected, often exceeds the demand. In the periods of under-utilization, one would like to reclaim the energy and energy costs whenever and wherever possible.
One option for saving energy and energy costs is to identify the under-utilized machines and turn them off. The disadvantage of this option is the extremely long latency incurred upon resuming the machine when it is found sometime later, or even worse, shortly later, that demand has surged. The long latency is attributed to the boot-up sequence which the machine must perform, including boot up of the operating system and hypervisor. In computing, a hypervisor, also called virtual machine manager (VMM), is one of many hardware virtualization techniques allowing multiple operating systems, typically termed “guests”, to run concurrently on a host computer. The hypervisor presents to the guest operating systems a virtual operating platform and manages the execution of the guest operating systems. This boot-up process can take on the order of minutes. This long latency prevents the environment from responding in an agile way to computing demand.
The S3 and S4 sleep modes are better alternatives than powering down a computing system completely because the time to resume the machine is shorter. When a machine is put into a sleep state, hardware components including the central processing unit (CPU) are placed in a low-power state, thus saving considerable energy and energy costs. The machine resumes slower from S4 than from S3 because it must re-populate memory by reading the needed data from disk storage and writing that data into memory. This is necessary because during S4 sleep, memory is not powered and therefore loses its state.
The S3 sleep mode is the fastest to resume because memory remains powered while other components are placed in low-power states. The system state is retained and therefore memory does not need to be re-populated, which saves time on resume. However, the S3 sleep mode consumes more energy than S4.
Therefore, a need exists for reduced power consumption when in the sleep mode such that S3 energy-efficiency can be brought closer to that of S4, thereby retaining all of the low-latency resume benefits provided by S3 sleep, but also reducing energy waste and energy costs to close the gap with S4.